This invention relates to synthetic lubricant base stock and, in particular, to a synthetic lubricant ester base stock having a decreased tendency to form deposits in gas turbine engines.
Synthetic ester base stocks for use in lubricants for gas turbine engines are well known. The base stocks are combined with standard lubricant additive packages to form the lubricant. In order for the lubricant to have properties suitable for use in the engine, the base stock must have certain physical properties. For example, it is desirable that the lubricant meet the specifications of the bearing rig test referred to in military specification MIL-L-23699C. The MIL-L-23699C specification also requires that the viscosity of the lubricant at 210.degree. F. be at least 5.0 centistokes (cSt) and the viscosity at -40.degree. F. must be less than 13,000 cSt. The highest allowable pour point for the lubricant is -65.degree. F. The pour point is the lowest temperature at which the base stock will flow as a liquid.
In general, synthetic gas turbine engine lubricants use esters as a base stock. The esters are a mixture of monopentaerythritol esters and dipentaerythritol esters. Alternatively, a mixture of trimethylolpropane esters and dipentaerythritol esters are used. The presence of the dipentaerythritol esters is generally considered necessary for imparting the required viscosity characteristics to the lubricant.
For example, U.S. Pat. No. 3,694,382, issued to Kleiman et al on Sept. 26, 1972, discloses an ester blend for use as a synthetic lubricant. The ester blend includes esters of trimethylolpropane and dipentaerythritol formed from a mixture of aliphatic monocarboxylic acids.
U.S. Pat. No. 4,049,563, issued to Burrous on Sept. 20, 1977, discloses a jet engine oil consisting of an ester of a C.sub.4 -C.sub.12 monocarboxylic acid, a polyol selected from pentaerythritol, dipentaerythritol, tripentaerythritol, trimethylolpropane, trimethylolmethane, trimethylolbutane, neopentylglycol and mixtures thereof and a soluble methyl phenyl polysiloxane.
U.S. Pat. No. 4,064,058, issued to Walker on Dec. 20, 1977, discloses a grease base stock including a blend of a normally liquid pentaerythritol ester product and a neopentyl glycol ester product.
U.S. Pat. No. 3,360,465 issued to Warman on Dec. 26, 1967 discloses synthetic ester lubricant compositions of pentaerythritol mixed esters. The pentaerythritol utilized includes at least 1.5 weight percent dipentaerythritol. The acid includes a mixture of from two to six monocarboxylic alkanoic acids having from five to nine carbon atoms with some of the lower acids being branched chain.
Every lubricant has a characteristic tendency to form deposits when used in an engine. If the deposits are excessive they will detract from the operating efficiency of the engine and create problems such as filter plugging. It has been found that esters formed from dipentaerythritol have a greater tendency to form deposits in gas turbine engines than esters formed from monopentaerythritol and trimethylolpropane. This is true whether or not the dipentaerythritOl esters are present alone or in combination with monopentaerythritol esters and/or trimethylolpropane esters.
Accordingly, it is desirable to provide a synthetic lubricant base stock which provides lubricants having acceptable viscosity and pour point characteristics without including esters of dipentaerythritol.